feat(gitea): support extra outbound HTTP headers via GITEA_EXTRA_HEADERS

Add a GITEA_EXTRA_HEADERS environment variable that accepts a JSON
object of header name/value pairs (e.g. Cloudflare Access
credentials) and applies them to every outbound request to Gitea,
both the raw pkg/gitea.DoJSON/DoBytes path and the SDK-backed
pkg/gitea.NewClient path, without overriding Authorization,
Content-Type, or Accept.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

pkg/flag/flag.go

@@ -1,5 +1,7 @@
 package flag
 
+import "net/http"
+
 var (
 	Host    string
 	Bind    string
@@ -15,4 +17,5 @@ var (
 	Debug         bool
 	AllowedTools  map[string]struct{}
 	AllowedScopes map[string]struct{}
+	ExtraHeaders  http.Header
 )